Internal combustion engines may utilize exhaust gas recirculation (EGR) in order to reduce emissions such as nitrogen oxides (NOx). Although EGR may be beneficial, an inaccurate determination of the amount of EGR may be detrimental to overall engine performance, resulting in increased emissions and decreased engine driveability.
One example approach for determining an amount of EGR is described in U.S. Pat. No. 5,145,566. In this example, EGR flow is determined based on humidity during conditions in which the engine is operating with a stoichiometric air-fuel ratio. Specifically, an exhaust gas sensor is employed to measure a relative amount of water vapor in an exhaust gas/intake air mixture as a relative amount of water in the exhaust gas/intake air mixture is dependent on the relative amount of EGR. Additionally, a second sensor is employed to determine a relative amount of water vapor in the air surrounding the vehicle (e.g., ambient water vapor). In this way, the amount of EGR may be accurately determined by adjusting the first measurement based on the relative amount of ambient water vapor.
During engine operation, however, the air-fuel ratio may fluctuate from stoichiometry (e.g., it may become lean or rich). Under such conditions, the relationship between exhaust gas recirculation and humidity changes due to variations in products of combustion. As such, the above method may produce an erroneous estimate of the amount of exhaust gas recirculation resulting in degraded engine performance including a loss of power and increased emissions. Further, the above method assumes a fuel of 100% gasoline and does not account for varying amounts of alcohol in the fuel (e.g., E85, which is 85% ethanol and 15% gasoline). Fuel blends comprising varying amounts of alcohol also affect the products of combustion which may lead to an inaccurate determination of exhaust gas recirculation when utilized by the above approach.
The inventors herein have recognized the above problems and have devised an approach to address them. Thus, in one example, a method for determining an amount of exhaust gas recirculation based on humidity with regard for variable parameters such as air-fuel ratio and fuel blend is disclosed. The method comprises determining an exhaust gas recirculation amount based on a first humidity generated from a first humidity sensor in an intake manifold of an engine and a second humidity generated from a second humidity sensor in an exhaust manifold of the engine. A third humidity sensor may also be positioned outside of the engine in order to generate an ambient humidity.
In this way, an accurate exhaust gas recirculation amount may be determined based on a balance of an ambient water mass and an exhaust water mass in the intake manifold. For example, an indication of the amount of exhaust gas in the intake air may be generated once the intake, exhaust, and ambient humidities are determined as the intake air may be composed of exhaust gas and ambient air. Therefore, accurate EGR amounts can be determined even when the combustion air-fuel ratio varies widely and/or the fuel type varies widely, since the EGR determination is based on an approach that has low sensitivity to these factors. After an accurate EGR amount is determined, various engine operating parameters may be adjusted in order to maintain or increase engine efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.